1. Field of the Invention
The present invention relates to predictive coding and decoding devices and, more particularly, to coding and decoding devices in a system for transmitting a so-called predictive coded code.
2. Related Background Art
Various coding schemes have been proposed to reduce the volume of transmission data when information such as image and audio data is transmitted in the form of a digital signal. One of the coding schemes is a differential pulse-code modulation (to be referred to as a DPCM hereinafter) for compressing information by using a correlation between adjacent sample values. According to DPCM, as is well-known, a coded sample value is temporarily decoded, a predictive value for a sample to be coded next is obtained by using the decoded value, and the between this predictive value and an actual sample value is quantized to perform coding.
FIG. 1 is block diagram showing the simplest arrangement of a DPCM coding device for performing prediction using a previous value. A sample value Xi input to an input terminal 1 is applied to a subtracter 2 to subtract a predictive value (a previous decoded value in this case) to be described later. A difference value output from the subtracter 2 is quantized by a quantizer 3. The quantized value is output to an output terminal 8 as a DPCM code Yi. The DPCM code Yi is also applied to an inverse quantizer 4. The inverse quantizer 4 decodes the DPCM code Yi into a difference value, and the difference value is- applied to an adder 5. The adder 5 adds the previous predictive value applied to the subtracter 2, thereby restoring the difference value as the original sample value. A limiter 6 limits the amplitude of an output from the adder 5 into a predetermined range, and an output from the limiter 6 is supplied to a D flip-flop 7. An output from the D flip-flop 7 serves as a predictive value of the next sample value. This value is supplied to the adder 5.
Small differences are normally concentrated in a distribution of difference values between the previous predictive values and the corresponding actual sample values. The difference value is coded and transmitted to achieve data compression transmission.
In order to increase the processing speed of the above predictive coding apparatus and simplify the circuit arrangement, the present applicant proposed to arrange all of the main part of the predictive coding device by look-up tables (U.S. application Ser. No. 288,994).
FIG. 2 is a block diagram showing an arrangement of a DPCM coding device whose main part is constituted by a ROM (Read-Only Memory). The circuit arrangement in FIG. 2 includes an input terminal 40 for a sample value Xi, a ROM 41 having a table for storing a predictive coded code Yi and a decoded value Xi, a D flip-flop 44 for converting the decoded value Xi (local decoded value) output from the ROM 41 into a previous predictive value Xi, a sync affix circuit 42 for adding sync data and identification (ID) data to a DPCM code output from the ROM 41, and an output terminal 43 for outputting a data string added with the DPCM code, sync data, and ID data.
The input sample value Xi (8 bits) from the input terminal 40 and the 8-bit predictive value Xi output from the D flip-flop 44 are supplied to address inputs of the ROM 41. More specifically, the 8-bit input sample value Xi and the 8-bit predictive value Xi serve as a 16-bit address input to the ROM 41. The ROM 41 initially stores a local coded value and a DPCM code as a combination of the input value Xi and the predictive value Xi. The DPCM code is supplied to the sync affix circuit 42, and the local decoded value is supplied to the D flip-flop 44. That is, the ROM 44 in FIG. 5 performs the processing corresponding to a portion 9 in FIG. 1.
FIG. 3 is a block diagram of an arrangement of a decoding device corresponding to the coding device shown in FIG. 2. This circuit arrangement includes an input terminal 50 for receiving the DPCM code, the sync information, and the ID code, a sync separation circuit 52 for separating the DPCM code from the sync and ID data, a ROM 51 for receiving a DPCM code Yi (e.g., 4 bits) and a predictive value (previous decoded value) Xi (to be described later) as address signals and outputting a decoded value Xi, and a D flip-flop 54 for converting the decoded value Xi output from the ROM 51 into the predictive value Xi.
The table of the ROM 51 is determined in correspondence with the table of the transmitting ROM 41. That is, the ROM 51 performs the processing corresponding to a portion 9a of FIG. 1. The sync data and the ID data separated by the sync separation circuit 52 are supplied to a control circuit 55, and the control circuit 55 controls the timings of the overall system.
With the arrangements of the coding and decoding devices described above, high-speed DPCM processing can be performed, and hardware can also be simplified.
The DPCM coding and decoding processing have different optimal processing characteristics in accordance with characteristics of data to be coded, which data are exemplified by image and audio data. Different processing characteristics must often be used in one image or one dynamic image program. In this manner, when data having different characteristics are to be processed, the coding device (FIG. 2) and the decoding device (FIG. 3) are not often suitable for the DPCM processing characteristics. In this case, quantization noise and the like are undesirably increased. Thus, there is much room for improvement in the coding and decoding devices in terms of adaptability for coded information.